1. Field of the Invention
The present invention relates to a nonlinear digital control circuit and method for a DC/DC converter.
2. Discussion of the Related Art
In particular, and in a non-limiting way, the present invention relates to a control circuit usable in Voltage Regulator Module (VRM) applications, wherein the converter is used for supplying loads that absorb high current in a discontinuous way, and thus have a rapidly varying load. A typical VRM application is the supply of computer processors, which, according to the operations to be executed, require currents varying rapidly (in a few microseconds) from a few microamps to 100-120 A.
Currently, the most widespread control circuits are of an analog type, but circuits of a digital type are increasingly spreading since they have numerous advantages, such as lower sensitivity to disturbance, environmental variations, and variations of parameters and can be used in different applications, without any need for modifications.
The structure of a typical digital DC/DC converter of a step-down type that can be used for VRM applications is illustrated in FIG. 1. The converter 1 of FIG. 1 comprises a power switch S, for example a MOS transistor, having a first conduction terminal connected to a first pin of an inductor L. The second pin of the inductor L is connected to a first pin of a capacitor C, parallel-connected to a load 2, for example a microprocessor. A diode D is coupled between the first pin of the inductor L and the second pin of the capacitor C and forms, with the inductor L and the capacitor C, a filter F. A second conduction terminal of the power switch S receives a d.c. voltage VS of, for example, 12 V.
The voltage VO on the load 2 is supplied to a linear control stage 3, including an adder node 4, which receives the output voltage VO, converted into digital format by a sampler/converter CONV A/D, and a reference voltage Vref and outputs an error signal Verr. The linear control stage 3 further comprises, cascade-connected to each other, an error amplifier EA receiving the error signal Verr, a control block, for example of a PID (Proportional-Integral-Derivative) type, a digital-to-analog converter DAC, and a PWM comparator, which receives, on a negative input, a ramp voltage W. The output X of the linear control stage 3 controls a driving circuit DR, which drives the gate terminal of the power switch S and causes switching-on and switching-off thereof.
When the load 2 varies, the output voltage VO undergoes a variation of opposite sign, which is detected by the linear control stage 3. This consequently modifies the on/off time (duty cycle) of the power switch S so as to bring the output voltage VO back to the steady-state value.
Notwithstanding the advantages, indicated above, of the digital technique as compared to the analog one, the digital technique is not readily applicable where the response speed of the converter is an essential requisite, as in the VRM applications mentioned above. In fact, the need to convert the input and output signals of the control circuit from analog to digital and vice versa and to numerically process the digital signals causes the digital technique to be intrinsically slower. On the other hand, for VRM applications, the system must be able to respond to extremely fast load variations, maintaining the output voltage within a preset range, in any operation condition. Here, identification of an appropriate corrective action to be inserted in the feedback line to increase the bandwidth of the system, and hence the speed of response, is not straightforward. In fact, this system is affected by the problem of the limit cycle, which is a phenomenon of instability that causes a considerable deterioration of the output-voltage waveform due to the difference between the resolutions of the A/D and D/A converters. The effects of the limit cycle can be reduced by reducing the gain, which however entails a reduction in the response speed.
Furthermore, in order to reduce the positive or negative voltage peaks when the load is rapidly modified, a solution has been proposed, referred to as AVP (Adaptive Voltage Positioning), which enables a reduction in the output-voltage swing in presence of load variations (see Modelling and simulation of new digital control for power conversion systems, G. Capponi, P. Livreri, M. Minieri, F. Marino; Electronics Specialists Conference, 2002, pesc. 02.2002 IEEE 33rd Annual, Volume 1, 2002, pp. 155-158). This solution envisages a variation in the reference voltage so as to exploit the entire tolerance window of the output voltage. In practice, upon detection of a high variation in output voltage or output current, the reference value is reduced or increased (according to whether there is an increase or a reduction in load) by a corrective value proportional to the maximum admissible current variation. Alternatively, for simplicity, the reference value can remain constant and the output voltage can be shifted up or down (in a direction opposite to the one expected for the reference voltage) by a quantity equal to the corrective value.
Using the AVP technique, the range of swing of the output voltage is reduced. However, the introduction of the AVP in the voltage-regulator circuit complicates the solution of the problem of the response speed.
In the literature, the problem of the response speed of a converter of the type indicated is generally solved by modifying the control loop or the converter topology (for instance, using a multiphase converter).
For example, A. Barrado, R. Vàzquez, A. Làzaro, J. Vàzquez, E. Olías <<New DC/DC Converter With Low Output Voltage And Fast Transient Response>>, 2003 IEEE, pp. 432-437, describes an example of nonlinear control inserted in a single-phase system. The solution proposed in this article comprises two DC/DC converters, one of which intervenes exclusively during load variations. By appropriately sizing the phase offset between the two converters, it is possible to obtain an improvement in the transient response. This solution has the considerable disadvantage that a converter is used only in presence of transients. In addition, from an analysis of the simulations, it may be inferred that the solution disclosed in this article would be far from effective in presence of AVP, in so far as the overshoots would completely nullify the advantages of the AVP. Furthermore, the solution has been presented for current variations not greater than 16 A, and consequently it is not applicable to situations where higher current variations (up to 80 A and above) are present.
Other solutions, such as bang-bang control (see, for example, U.S. 2002/0048180) are aimed at reducing the recovery time, seeking to maintain balance between the currents of various DC/DC converters operating in parallel. This technique is apparently not compatible with AVP and enables a certain hysteresis on the output voltage. In practice, this technique enables maximization of the response speed, causing an increase or decrease in the output current as fast as possible and is not concerned with oscillations on the output voltage.
The aim of the present invention is therefore to solve the problems that afflict known solutions, providing a control circuit of a digital type, which has a high response speed to load variations, reducing as much as possible overshooting and undershooting of the output voltage.
According to the present invention, a control circuit for a DC/DC converter is provided, comprising a linear-control loop, which has an input receiving a quantity to be controlled and a first reference quantity, and has an output supplying a modulation value, and a nonlinear modulation unit for said first reference quantity, said nonlinear modulation unit being activated in presence of a first variation of said quantity to be controlled higher than an intervention threshold.
The invention also provides a method for controlling a DC/DC converter receiving a modulation value and supplying a quantity to be controlled, comprising: acquiring said quantity to be controlled and a first reference quantity; regulating with a closed linear loop said quantity to be controlled on the basis of said first reference quantity; and detecting a first variation of said quantity to be controlled higher than an intervention threshold, and modulating, in a nonlinear way, said first reference quantity in presence of said first variation of said quantity to be controlled.